1 2 Linux kernel coding style
3 4This is a short document describing the preferred coding style for the
5linux kernel. Coding style is very personal, and I won't _force_ my
6views on anybody, but this is what goes for anything that I have to be
7able to maintain, and I'd prefer it for most other things too. Please
8at least consider the points made here.
9 10First off, I'd suggest printing out a copy of the GNU coding standards,
11and NOT read it. Burn them, it's a great symbolic gesture.
12 13Anyway, here goes:
14 15 16 Chapter 1: Indentation
17 18Tabs are 8 characters, and thus indentations are also 8 characters.
19There are heretic movements that try to make indentations 4 (or even 2!)
20characters deep, and that is akin to trying to define the value of PI to
21be 3.
22 23Rationale: The whole idea behind indentation is to clearly define where
24a block of control starts and ends. Especially when you've been looking
25at your screen for 20 straight hours, you'll find it a lot easier to see
26how the indentation works if you have large indentations.
27 28Now, some people will claim that having 8-character indentations makes
29the code move too far to the right, and makes it hard to read on a
3080-character terminal screen. The answer to that is that if you need
31more than 3 levels of indentation, you're screwed anyway, and should fix
32your program.
33 34In short, 8-char indents make things easier to read, and have the added
35benefit of warning you when you're nesting your functions too deep.
36Heed that warning.
37 38The preferred way to ease multiple indentation levels in a switch statement is
39to align the "switch" and its subordinate "case" labels in the same column
40instead of "double-indenting" the "case" labels. E.g.:
41 42 switch (suffix) {
43 case 'G':
44 case 'g':
45 mem <<= 30;
46 break;
47 case 'M':
48 case 'm':
49 mem <<= 20;
50 break;
51 case 'K':
52 case 'k':
53 mem <<= 10;
54 /* fall through */
55 default:
56 break;
57 }
58 59 60Don't put multiple statements on a single line unless you have
61something to hide:
62 63 if (condition) do_this;
64 do_something_everytime;
65 66Don't put multiple assignments on a single line either. Kernel coding style
67is super simple. Avoid tricky expressions.
68 69Outside of comments, documentation and except in Kconfig, spaces are never
70used for indentation, and the above example is deliberately broken.
71 72Get a decent editor and don't leave whitespace at the end of lines.
73 74 75 Chapter 2: Breaking long lines and strings
76 77Coding style is all about readability and maintainability using commonly
78available tools.
79 80The limit on the length of lines is 80 columns and this is a strongly
81preferred limit.
82 83Statements longer than 80 columns will be broken into sensible chunks, unless
84exceeding 80 columns significantly increases readability and does not hide
85information. Descendants are always substantially shorter than the parent and
86are placed substantially to the right. The same applies to function headers
87with a long argument list. However, never break user-visible strings such as
88printk messages, because that breaks the ability to grep for them.
89 90 91 Chapter 3: Placing Braces and Spaces
92 93The other issue that always comes up in C styling is the placement of
94braces. Unlike the indent size, there are few technical reasons to
95choose one placement strategy over the other, but the preferred way, as
96shown to us by the prophets Kernighan and Ritchie, is to put the opening
97brace last on the line, and put the closing brace first, thusly:
98 99 if (x is true) {
100 we do y
101 }
102 103This applies to all non-function statement blocks (if, switch, for,
104while, do). E.g.:
105 106 switch (action) {
107 case KOBJ_ADD:
108 return "add";
109 case KOBJ_REMOVE:
110 return "remove";
111 case KOBJ_CHANGE:
112 return "change";
113 default:
114 return NULL;
115 }
116 117However, there is one special case, namely functions: they have the
118opening brace at the beginning of the next line, thus:
119 120 int function(int x)
121 {
122 body of function
123 }
124 125Heretic people all over the world have claimed that this inconsistency
126is ... well ... inconsistent, but all right-thinking people know that
127(a) K&R are _right_ and (b) K&R are right. Besides, functions are
128special anyway (you can't nest them in C).
129 130Note that the closing brace is empty on a line of its own, _except_ in
131the cases where it is followed by a continuation of the same statement,
132ie a "while" in a do-statement or an "else" in an if-statement, like
133this:
134 135 do {
136 body of do-loop
137 } while (condition);
138 139and
140 141 if (x == y) {
142 ..
143 } else if (x > y) {
144 ...
145 } else {
146 ....
147 }
148 149Rationale: K&R.
150 151Also, note that this brace-placement also minimizes the number of empty
152(or almost empty) lines, without any loss of readability. Thus, as the
153supply of new-lines on your screen is not a renewable resource (think
15425-line terminal screens here), you have more empty lines to put
155comments on.
156 157Do not unnecessarily use braces where a single statement will do.
158 159if (condition)
160 action();
161 162and
163 164if (condition)
165 do_this();
166else
167 do_that();
168 169This does not apply if only one branch of a conditional statement is a single
170statement; in the latter case use braces in both branches:
171 172if (condition) {
173 do_this();
174 do_that();
175} else {
176 otherwise();
177}
178 179 3.1: Spaces
180 181Linux kernel style for use of spaces depends (mostly) on
182function-versus-keyword usage. Use a space after (most) keywords. The
183notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
184somewhat like functions (and are usually used with parentheses in Linux,
185although they are not required in the language, as in: "sizeof info" after
186"struct fileinfo info;" is declared).
187 188So use a space after these keywords:
189 if, switch, case, for, do, while
190but not with sizeof, typeof, alignof, or __attribute__. E.g.,
191 s = sizeof(struct file);
192 193Do not add spaces around (inside) parenthesized expressions. This example is
194*bad*:
195 196 s = sizeof( struct file );
197 198When declaring pointer data or a function that returns a pointer type, the
199preferred use of '*' is adjacent to the data name or function name and not
200adjacent to the type name. Examples:
201 202 char *linux_banner;
203 unsigned long long memparse(char *ptr, char **retptr);
204 char *match_strdup(substring_t *s);
205 206Use one space around (on each side of) most binary and ternary operators,
207such as any of these:
208 209 = + - < > * / % | & ^ <= >= == != ? :
210 211but no space after unary operators:
212 & * + - ~ ! sizeof typeof alignof __attribute__ defined
213 214no space before the postfix increment & decrement unary operators:
215 ++ --
216 217no space after the prefix increment & decrement unary operators:
218 ++ --
219 220and no space around the '.' and "->" structure member operators.
221 222Do not leave trailing whitespace at the ends of lines. Some editors with
223"smart" indentation will insert whitespace at the beginning of new lines as
224appropriate, so you can start typing the next line of code right away.
225However, some such editors do not remove the whitespace if you end up not
226putting a line of code there, such as if you leave a blank line. As a result,
227you end up with lines containing trailing whitespace.
228 229Git will warn you about patches that introduce trailing whitespace, and can
230optionally strip the trailing whitespace for you; however, if applying a series
231of patches, this may make later patches in the series fail by changing their
232context lines.
233 234 235 Chapter 4: Naming
236 237C is a Spartan language, and so should your naming be. Unlike Modula-2
238and Pascal programmers, C programmers do not use cute names like
239ThisVariableIsATemporaryCounter. A C programmer would call that
240variable "tmp", which is much easier to write, and not the least more
241difficult to understand.
242 243HOWEVER, while mixed-case names are frowned upon, descriptive names for
244global variables are a must. To call a global function "foo" is a
245shooting offense.
246 247GLOBAL variables (to be used only if you _really_ need them) need to
248have descriptive names, as do global functions. If you have a function
249that counts the number of active users, you should call that
250"count_active_users()" or similar, you should _not_ call it "cntusr()".
251 252Encoding the type of a function into the name (so-called Hungarian
253notation) is brain damaged - the compiler knows the types anyway and can
254check those, and it only confuses the programmer. No wonder MicroSoft
255makes buggy programs.
256 257LOCAL variable names should be short, and to the point. If you have
258some random integer loop counter, it should probably be called "i".
259Calling it "loop_counter" is non-productive, if there is no chance of it
260being mis-understood. Similarly, "tmp" can be just about any type of
261variable that is used to hold a temporary value.
262 263If you are afraid to mix up your local variable names, you have another
264problem, which is called the function-growth-hormone-imbalance syndrome.
265See chapter 6 (Functions).
266 267 268 Chapter 5: Typedefs
269 270Please don't use things like "vps_t".
271 272It's a _mistake_ to use typedef for structures and pointers. When you see a
273 274 vps_t a;
275 276in the source, what does it mean?
277 278In contrast, if it says
279 280 struct virtual_container *a;
281 282you can actually tell what "a" is.
283 284Lots of people think that typedefs "help readability". Not so. They are
285useful only for:
286 287 (a) totally opaque objects (where the typedef is actively used to _hide_
288 what the object is).
289 290 Example: "pte_t" etc. opaque objects that you can only access using
291 the proper accessor functions.
292 293 NOTE! Opaqueness and "accessor functions" are not good in themselves.
294 The reason we have them for things like pte_t etc. is that there
295 really is absolutely _zero_ portably accessible information there.
296 297 (b) Clear integer types, where the abstraction _helps_ avoid confusion
298 whether it is "int" or "long".
299 300 u8/u16/u32 are perfectly fine typedefs, although they fit into
301 category (d) better than here.
302 303 NOTE! Again - there needs to be a _reason_ for this. If something is
304 "unsigned long", then there's no reason to do
305 306 typedef unsigned long myflags_t;
307 308 but if there is a clear reason for why it under certain circumstances
309 might be an "unsigned int" and under other configurations might be
310 "unsigned long", then by all means go ahead and use a typedef.
311 312 (c) when you use sparse to literally create a _new_ type for
313 type-checking.
314 315 (d) New types which are identical to standard C99 types, in certain
316 exceptional circumstances.
317 318 Although it would only take a short amount of time for the eyes and
319 brain to become accustomed to the standard types like 'uint32_t',
320 some people object to their use anyway.
321 322 Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
323 signed equivalents which are identical to standard types are
324 permitted -- although they are not mandatory in new code of your
325 own.
326 327 When editing existing code which already uses one or the other set
328 of types, you should conform to the existing choices in that code.
329 330 (e) Types safe for use in userspace.
331 332 In certain structures which are visible to userspace, we cannot
333 require C99 types and cannot use the 'u32' form above. Thus, we
334 use __u32 and similar types in all structures which are shared
335 with userspace.
336 337Maybe there are other cases too, but the rule should basically be to NEVER
338EVER use a typedef unless you can clearly match one of those rules.
339 340In general, a pointer, or a struct that has elements that can reasonably
341be directly accessed should _never_ be a typedef.
342 343 344 Chapter 6: Functions
345 346Functions should be short and sweet, and do just one thing. They should
347fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
348as we all know), and do one thing and do that well.
349 350The maximum length of a function is inversely proportional to the
351complexity and indentation level of that function. So, if you have a
352conceptually simple function that is just one long (but simple)
353case-statement, where you have to do lots of small things for a lot of
354different cases, it's OK to have a longer function.
355 356However, if you have a complex function, and you suspect that a
357less-than-gifted first-year high-school student might not even
358understand what the function is all about, you should adhere to the
359maximum limits all the more closely. Use helper functions with
360descriptive names (you can ask the compiler to in-line them if you think
361it's performance-critical, and it will probably do a better job of it
362than you would have done).
363 364Another measure of the function is the number of local variables. They
365shouldn't exceed 5-10, or you're doing something wrong. Re-think the
366function, and split it into smaller pieces. A human brain can
367generally easily keep track of about 7 different things, anything more
368and it gets confused. You know you're brilliant, but maybe you'd like
369to understand what you did 2 weeks from now.
370 371In source files, separate functions with one blank line. If the function is
372exported, the EXPORT* macro for it should follow immediately after the closing
373function brace line. E.g.:
374 375int system_is_up(void)
376{
377 return system_state == SYSTEM_RUNNING;
378}
379EXPORT_SYMBOL(system_is_up);
380 381In function prototypes, include parameter names with their data types.
382Although this is not required by the C language, it is preferred in Linux
383because it is a simple way to add valuable information for the reader.
384 385 386 Chapter 7: Centralized exiting of functions
387 388Albeit deprecated by some people, the equivalent of the goto statement is
389used frequently by compilers in form of the unconditional jump instruction.
390 391The goto statement comes in handy when a function exits from multiple
392locations and some common work such as cleanup has to be done.
393 394The rationale is:
395 396- unconditional statements are easier to understand and follow
397- nesting is reduced
398- errors by not updating individual exit points when making
399 modifications are prevented
400- saves the compiler work to optimize redundant code away ;)
401 402int fun(int a)
403{
404 int result = 0;
405 char *buffer = kmalloc(SIZE);
406 407 if (buffer == NULL)
408 return -ENOMEM;
409 410 if (condition1) {
411 while (loop1) {
412 ...
413 }
414 result = 1;
415 goto out;
416 }
417 ...
418out:
419 kfree(buffer);
420 return result;
421}
422 423 Chapter 8: Commenting
424 425Comments are good, but there is also a danger of over-commenting. NEVER
426try to explain HOW your code works in a comment: it's much better to
427write the code so that the _working_ is obvious, and it's a waste of
428time to explain badly written code.
429 430Generally, you want your comments to tell WHAT your code does, not HOW.
431Also, try to avoid putting comments inside a function body: if the
432function is so complex that you need to separately comment parts of it,
433you should probably go back to chapter 6 for a while. You can make
434small comments to note or warn about something particularly clever (or
435ugly), but try to avoid excess. Instead, put the comments at the head
436of the function, telling people what it does, and possibly WHY it does
437it.
438 439When commenting the kernel API functions, please use the kernel-doc format.
440See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
441for details.
442 443Linux style for comments is the C89 "/* ... */" style.
444Don't use C99-style "// ..." comments.
445 446The preferred style for long (multi-line) comments is:
447 448 /*
449 * This is the preferred style for multi-line
450 * comments in the Linux kernel source code.
451 * Please use it consistently.
452 *
453 * Description: A column of asterisks on the left side,
454 * with beginning and ending almost-blank lines.
455 */
456 457For files in net/ and drivers/net/ the preferred style for long (multi-line)
458comments is a little different.
459 460 /* The preferred comment style for files in net/ and drivers/net
461 * looks like this.
462 *
463 * It is nearly the same as the generally preferred comment style,
464 * but there is no initial almost-blank line.
465 */
466 467It's also important to comment data, whether they are basic types or derived
468types. To this end, use just one data declaration per line (no commas for
469multiple data declarations). This leaves you room for a small comment on each
470item, explaining its use.
471 472 473 Chapter 9: You've made a mess of it
474 475That's OK, we all do. You've probably been told by your long-time Unix
476user helper that "GNU emacs" automatically formats the C sources for
477you, and you've noticed that yes, it does do that, but the defaults it
478uses are less than desirable (in fact, they are worse than random
479typing - an infinite number of monkeys typing into GNU emacs would never
480make a good program).
481 482So, you can either get rid of GNU emacs, or change it to use saner
483values. To do the latter, you can stick the following in your .emacs file:
484 485(defun c-lineup-arglist-tabs-only (ignored)
486 "Line up argument lists by tabs, not spaces"
487 (let* ((anchor (c-langelem-pos c-syntactic-element))
488 (column (c-langelem-2nd-pos c-syntactic-element))
489 (offset (- (1+ column) anchor))
490 (steps (floor offset c-basic-offset)))
491 (* (max steps 1)
492 c-basic-offset)))
493 494(add-hook 'c-mode-common-hook
495 (lambda ()
496 ;; Add kernel style
497 (c-add-style
498 "linux-tabs-only"
499 '("linux" (c-offsets-alist
500 (arglist-cont-nonempty
501 c-lineup-gcc-asm-reg
502 c-lineup-arglist-tabs-only))))))
503 504(add-hook 'c-mode-hook
505 (lambda ()
506 (let ((filename (buffer-file-name)))
507 ;; Enable kernel mode for the appropriate files
508 (when (and filename
509 (string-match (expand-file-name "~/src/linux-trees")
510 filename))
511 (setq indent-tabs-mode t)
512 (c-set-style "linux-tabs-only")))))
513 514This will make emacs go better with the kernel coding style for C
515files below ~/src/linux-trees.
516 517But even if you fail in getting emacs to do sane formatting, not
518everything is lost: use "indent".
519 520Now, again, GNU indent has the same brain-dead settings that GNU emacs
521has, which is why you need to give it a few command line options.
522However, that's not too bad, because even the makers of GNU indent
523recognize the authority of K&R (the GNU people aren't evil, they are
524just severely misguided in this matter), so you just give indent the
525options "-kr -i8" (stands for "K&R, 8 character indents"), or use
526"scripts/Lindent", which indents in the latest style.
527 528"indent" has a lot of options, and especially when it comes to comment
529re-formatting you may want to take a look at the man page. But
530remember: "indent" is not a fix for bad programming.
531 532 533 Chapter 10: Kconfig configuration files
534 535For all of the Kconfig* configuration files throughout the source tree,
536the indentation is somewhat different. Lines under a "config" definition
537are indented with one tab, while help text is indented an additional two
538spaces. Example:
539 540config AUDIT
541 bool "Auditing support"
542 depends on NET
543 help
544 Enable auditing infrastructure that can be used with another
545 kernel subsystem, such as SELinux (which requires this for
546 logging of avc messages output). Does not do system-call
547 auditing without CONFIG_AUDITSYSCALL.
548 549Features that might still be considered unstable should be defined as
550dependent on "EXPERIMENTAL":
551 552config SLUB
553 depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT
554 bool "SLUB (Unqueued Allocator)"
555 ...
556 557while seriously dangerous features (such as write support for certain
558filesystems) should advertise this prominently in their prompt string:
559 560config ADFS_FS_RW
561 bool "ADFS write support (DANGEROUS)"
562 depends on ADFS_FS
563 ...
564 565For full documentation on the configuration files, see the file
566Documentation/kbuild/kconfig-language.txt.
567 568 569 Chapter 11: Data structures
570 571Data structures that have visibility outside the single-threaded
572environment they are created and destroyed in should always have
573reference counts. In the kernel, garbage collection doesn't exist (and
574outside the kernel garbage collection is slow and inefficient), which
575means that you absolutely _have_ to reference count all your uses.
576 577Reference counting means that you can avoid locking, and allows multiple
578users to have access to the data structure in parallel - and not having
579to worry about the structure suddenly going away from under them just
580because they slept or did something else for a while.
581 582Note that locking is _not_ a replacement for reference counting.
583Locking is used to keep data structures coherent, while reference
584counting is a memory management technique. Usually both are needed, and
585they are not to be confused with each other.
586 587Many data structures can indeed have two levels of reference counting,
588when there are users of different "classes". The subclass count counts
589the number of subclass users, and decrements the global count just once
590when the subclass count goes to zero.
591 592Examples of this kind of "multi-level-reference-counting" can be found in
593memory management ("struct mm_struct": mm_users and mm_count), and in
594filesystem code ("struct super_block": s_count and s_active).
595 596Remember: if another thread can find your data structure, and you don't
597have a reference count on it, you almost certainly have a bug.
598 599 600 Chapter 12: Macros, Enums and RTL
601 602Names of macros defining constants and labels in enums are capitalized.
603 604#define CONSTANT 0x12345
605 606Enums are preferred when defining several related constants.
607 608CAPITALIZED macro names are appreciated but macros resembling functions
609may be named in lower case.
610 611Generally, inline functions are preferable to macros resembling functions.
612 613Macros with multiple statements should be enclosed in a do - while block:
614 615#define macrofun(a, b, c) \
616 do { \
617 if (a == 5) \
618 do_this(b, c); \
619 } while (0)
620 621Things to avoid when using macros:
622 6231) macros that affect control flow:
624 625#define FOO(x) \
626 do { \
627 if (blah(x) < 0) \
628 return -EBUGGERED; \
629 } while(0)
630 631is a _very_ bad idea. It looks like a function call but exits the "calling"
632function; don't break the internal parsers of those who will read the code.
633 6342) macros that depend on having a local variable with a magic name:
635 636#define FOO(val) bar(index, val)
637 638might look like a good thing, but it's confusing as hell when one reads the
639code and it's prone to breakage from seemingly innocent changes.
640 6413) macros with arguments that are used as l-values: FOO(x) = y; will
642bite you if somebody e.g. turns FOO into an inline function.
643 6444) forgetting about precedence: macros defining constants using expressions
645must enclose the expression in parentheses. Beware of similar issues with
646macros using parameters.
647 648#define CONSTANT 0x4000
649#define CONSTEXP (CONSTANT | 3)
650 651The cpp manual deals with macros exhaustively. The gcc internals manual also
652covers RTL which is used frequently with assembly language in the kernel.
653 654 655 Chapter 13: Printing kernel messages
656 657Kernel developers like to be seen as literate. Do mind the spelling
658of kernel messages to make a good impression. Do not use crippled
659words like "dont"; use "do not" or "don't" instead. Make the messages
660concise, clear, and unambiguous.
661 662Kernel messages do not have to be terminated with a period.
663 664Printing numbers in parentheses (%d) adds no value and should be avoided.
665 666There are a number of driver model diagnostic macros in <linux/device.h>
667which you should use to make sure messages are matched to the right device
668and driver, and are tagged with the right level: dev_err(), dev_warn(),
669dev_info(), and so forth. For messages that aren't associated with a
670particular device, <linux/printk.h> defines pr_debug() and pr_info().
671 672Coming up with good debugging messages can be quite a challenge; and once
673you have them, they can be a huge help for remote troubleshooting. Such
674messages should be compiled out when the DEBUG symbol is not defined (that
675is, by default they are not included). When you use dev_dbg() or pr_debug(),
676that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG.
677A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the
678ones already enabled by DEBUG.
679 680 681 Chapter 14: Allocating memory
682 683The kernel provides the following general purpose memory allocators:
684kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
685vzalloc(). Please refer to the API documentation for further information
686about them.
687 688The preferred form for passing a size of a struct is the following:
689 690 p = kmalloc(sizeof(*p), ...);
691 692The alternative form where struct name is spelled out hurts readability and
693introduces an opportunity for a bug when the pointer variable type is changed
694but the corresponding sizeof that is passed to a memory allocator is not.
695 696Casting the return value which is a void pointer is redundant. The conversion
697from void pointer to any other pointer type is guaranteed by the C programming
698language.
699 700The preferred form for allocating an array is the following:
701 702 p = kmalloc_array(n, sizeof(...), ...);
703 704The preferred form for allocating a zeroed array is the following:
705 706 p = kcalloc(n, sizeof(...), ...);
707 708Both forms check for overflow on the allocation size n * sizeof(...),
709and return NULL if that occurred.
710 711 712 Chapter 15: The inline disease
713 714There appears to be a common misperception that gcc has a magic "make me
715faster" speedup option called "inline". While the use of inlines can be
716appropriate (for example as a means of replacing macros, see Chapter 12), it
717very often is not. Abundant use of the inline keyword leads to a much bigger
718kernel, which in turn slows the system as a whole down, due to a bigger
719icache footprint for the CPU and simply because there is less memory
720available for the pagecache. Just think about it; a pagecache miss causes a
721disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
722that can go into these 5 milliseconds.
723 724A reasonable rule of thumb is to not put inline at functions that have more
725than 3 lines of code in them. An exception to this rule are the cases where
726a parameter is known to be a compiletime constant, and as a result of this
727constantness you *know* the compiler will be able to optimize most of your
728function away at compile time. For a good example of this later case, see
729the kmalloc() inline function.
730 731Often people argue that adding inline to functions that are static and used
732only once is always a win since there is no space tradeoff. While this is
733technically correct, gcc is capable of inlining these automatically without
734help, and the maintenance issue of removing the inline when a second user
735appears outweighs the potential value of the hint that tells gcc to do
736something it would have done anyway.
737 738 739 Chapter 16: Function return values and names
740 741Functions can return values of many different kinds, and one of the
742most common is a value indicating whether the function succeeded or
743failed. Such a value can be represented as an error-code integer
744(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure,
745non-zero = success).
746 747Mixing up these two sorts of representations is a fertile source of
748difficult-to-find bugs. If the C language included a strong distinction
749between integers and booleans then the compiler would find these mistakes
750for us... but it doesn't. To help prevent such bugs, always follow this
751convention:
752 753 If the name of a function is an action or an imperative command,
754 the function should return an error-code integer. If the name
755 is a predicate, the function should return a "succeeded" boolean.
756 757For example, "add work" is a command, and the add_work() function returns 0
758for success or -EBUSY for failure. In the same way, "PCI device present" is
759a predicate, and the pci_dev_present() function returns 1 if it succeeds in
760finding a matching device or 0 if it doesn't.
761 762All EXPORTed functions must respect this convention, and so should all
763public functions. Private (static) functions need not, but it is
764recommended that they do.
765 766Functions whose return value is the actual result of a computation, rather
767than an indication of whether the computation succeeded, are not subject to
768this rule. Generally they indicate failure by returning some out-of-range
769result. Typical examples would be functions that return pointers; they use
770NULL or the ERR_PTR mechanism to report failure.
771 772 773 Chapter 17: Don't re-invent the kernel macros
774 775The header file include/linux/kernel.h contains a number of macros that
776you should use, rather than explicitly coding some variant of them yourself.
777For example, if you need to calculate the length of an array, take advantage
778of the macro
779 780 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
781 782Similarly, if you need to calculate the size of some structure member, use
783 784 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
785 786There are also min() and max() macros that do strict type checking if you
787need them. Feel free to peruse that header file to see what else is already
788defined that you shouldn't reproduce in your code.
789 790 791 Chapter 18: Editor modelines and other cruft
792 793Some editors can interpret configuration information embedded in source files,
794indicated with special markers. For example, emacs interprets lines marked
795like this:
796 797-*- mode: c -*-
798 799Or like this:
800 801/*
802Local Variables:
803compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
804End:
805*/
806 807Vim interprets markers that look like this:
808 809/* vim:set sw=8 noet */
810 811Do not include any of these in source files. People have their own personal
812editor configurations, and your source files should not override them. This
813includes markers for indentation and mode configuration. People may use their
814own custom mode, or may have some other magic method for making indentation
815work correctly.
816 817 818 Chapter 19: Inline assembly
819 820In architecture-specific code, you may need to use inline assembly to interface
821with CPU or platform functionality. Don't hesitate to do so when necessary.
822However, don't use inline assembly gratuitously when C can do the job. You can
823and should poke hardware from C when possible.
824 825Consider writing simple helper functions that wrap common bits of inline
826assembly, rather than repeatedly writing them with slight variations. Remember
827that inline assembly can use C parameters.
828 829Large, non-trivial assembly functions should go in .S files, with corresponding
830C prototypes defined in C header files. The C prototypes for assembly
831functions should use "asmlinkage".
832 833You may need to mark your asm statement as volatile, to prevent GCC from
834removing it if GCC doesn't notice any side effects. You don't always need to
835do so, though, and doing so unnecessarily can limit optimization.
836 837When writing a single inline assembly statement containing multiple
838instructions, put each instruction on a separate line in a separate quoted
839string, and end each string except the last with \n\t to properly indent the
840next instruction in the assembly output:
841 842 asm ("magic %reg1, #42\n\t"
843 "more_magic %reg2, %reg3"
844 : /* outputs */ : /* inputs */ : /* clobbers */);
845 846 847 848 Appendix I: References
849 850The C Programming Language, Second Edition
851by Brian W. Kernighan and Dennis M. Ritchie.
852Prentice Hall, Inc., 1988.
853ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
854URL: http://cm.bell-labs.com/cm/cs/cbook/ 855 856The Practice of Programming
857by Brian W. Kernighan and Rob Pike.
858Addison-Wesley, Inc., 1999.
859ISBN 0-201-61586-X.
860URL: http://cm.bell-labs.com/cm/cs/tpop/ 861 862GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
863gcc internals and indent, all available from http://www.gnu.org/manual/ 864 865WG14 is the international standardization working group for the programming
866language C, URL: http://www.open-std.org/JTC1/SC22/WG14/ 867 868Kernel CodingStyle, by greg@kroah.com at OLS 2002:
869http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ 870 871